The invention relates to pipettes and especially to pipettes that insure the discharge of the entire volume of liquid aspirated during the pipette filling operation.
In recent years manual pipettes have largely supplanted mouth pipettes in laboratory procedures. By the term manual pipette is meant a plunger type pipette which is used simply by driving a piston between two limiting positions to aspirate a liquid being pipetted into a reservoir, usually a disposable unit forming no part of the pipette proper, and thereafter to expel the liquid from the reservoir. Usually the depression and release of the piston will draw a quantity of liquid into the reservoir and a second depression of the piston will expel the liguid, although pipettes have been provided in which depression of the piston will fill the reservoir and release of the piston will expel the liquid. The reasons for the current popularity of manual pipettes are manifest. For example, the manual skill required to pipette a precise quantity of liquid is less demanding with manual pipettes since the precision is built into the pipette during the manufacture thereof. Moreover, since manual pipettes are generally used with disposable pipette tips into which the liquids are drawn, the pipette itself need not be cleaned or sterilized between uses since the liquid never is drawn into the pipette proper.
The utility of many laboratory procedures requires the precise measurement of a small volume of liquid. Thus, it is not uncommon to require the accurate dispensing of volumes of less than ten milliliters. Even for larger volumes it is desirable that the accuracy of the measurement be assured and that it be repeatable over many pipetting operations. The design of manual pipettes generally assures that a precise predetermined volume of liquid is aspirated into the pipette reservoir, but the discharges of that precise volume is not always achieved. Quite often, a small quantity of the liquid remains after the discharge stroke of the piston is completed. The liquid remaining may be a droplet formed at the orifice of the pipette tip which is not blown out with the rest of the liquid. This often results because the air in the reservoir above the liquid level is a compressible fluid that cannot positively expel all of the liquid when the pipette piston is depressed to expel the liquid. Attempts to minimize the consequences of the compressible air column lead to a pipette design that reduces the air volume between the piston and the liquid level in the reservoir. But this means that the piston is near the liquid level and perhaps will be contacted by the liquid in some operations. This, of course, is undesirable since cross contamination between samples being pipetted can occur. Also, the orifice of the pipette tip can be designed to minimize the formation of droplets of liquid thereon. Thin layers of liquid may ahdere to the walls of the liquid reservoir, i.e., the disposable pipette tip. This tendency is greatly reduced by the use of non-wetting plastics for pipette tips.
It will be appreciated from the foregoing, however, that the accurate measurement of liquids in manual pipettes is an important consideration in the use of such pipettes for critical laboratory procedures. This is especially true where small volumes, in the order of ten milliliters or less, are being measured. In such cases, the volume of a droplet remaining on a pipette tip will be an appreciable part of the volume initially aspirated. So much so has this become a consideration that a great deal of inventive effort has been directed towards the provision of pipettes that will expel essentially all of the liquid initially drawn into the pipette.
These efforts have generally resulted in a pipette having two different strokes, one for filling the pipette reservoir with a measured volume of a liquid, and a second longer stroke for discharging the liquid. In a manual pipette, the piston is usually spring biased to a normal or home position and moved from that position, against the force exerted by the spring, to a position determined by a stop. This movement expels a fixed volume of air from the pipette top so that when the piston is restored to its home position by the action of the spring a like volume of liquid is aspirated into the pipette tip. The liquid is then expelled by a second depression of the piston. If the stop referred to is not a fixed stop, but a stop formed by a relatively stiff spring, then the filling stroke of the piston would take place against the force exerted by a relatively light spring and would be arrested when the technician tactilely encountered the heavy spring. The second or expelling stroke would simply be made with sufficient force to overcome the effects of the stiff spring. It should be clear that such an arrangement requires discernment on the part of the technician since the accuracy of the pipetting operation depends on stopping the first or filling stroke the instant the stiff spring is felt. Any compression of the stiff spring on the filling stroke destroys the accuracy of the pipette. Obviously, such a solution is not satisfactory.
Others have overcome the deficiencies of the pipettes just alluded to by making the stop that controls the filling stroke a fixed stop. But then, before the discharge stroke is made, an element of the pipette is rotated so that the stop can be cleared or by-passed as the piston is driven a longer distance to discharge the liquid contained in the pipette tip. This, also, is an unsatisfactory solution since it imposes an additional burden on the technician in that he has to perform the awkward task of rotating the pipette piston before each discharge and filling stroke. When a large number of pipetting operations are to be performed, this becomes arduous and annoying.